1. Technical Field
The present invention relates to a scheduling method and apparatus, more particularly to scheduling that can reduce delays that may occur during the transmission of data in a mobile communication system of a massive MIMO environment that supports uplink multi-user transmission.
2. Description of the Related Art
In conventional mobile communication systems, including those that use LTE (long-term evolution), predicting the performance of a channel between a user terminal and a base station antenna can be very difficult or very inaccurate, due to channel fluctuations between the user terminal and the base station antenna.
In such an environment, it is crucial that user scheduling first performs the process of acquiring accurate channel information if the user is to be guaranteed high quality of service, and due to the inherent property of the channel changing within short durations of time in a mobile communication environment, a process of transmitting reference signals in short periods for the scheduling is inevitably required.
FIG. 1 is a flow diagram illustrating a user scheduling procedure according to the related art.
Incidentally, the procedure in FIG. 1 can apply to the cases of an uplink and to a TDD (time-division duplexing) downlink.
In FIG. 1, user terminals desiring scheduling may transmit reference signals (RS) agreed upon beforehand with the base station over an uplink (operation S101).
Here, the number of reference signals needed and the corresponding number of resource elements (RE) would be proportional to the number of user terminals desiring scheduling.
Incidentally, in the case of FDD (frequency-division duplexing), the base station transmits reference signals via downlinks, and thus the number of resource elements would increase in proportion to the number of antennas at the base station.
After operation S101, the base station may estimate the channel information of the user terminals by using the reference signals received from the user terminals, and may perform user scheduling based on the channel information (operation S102).
After operation S102, the base station may transmit control signals for the user terminals, of which scheduling has been determined, via a PDCCH (physical downlink control channel) (operation S103).
Here, the scheduling information transmitted to the user terminals can include the indexes of the scheduled user terminals, positions of the scheduled resource elements, uplink transmission power, etc.
After operation S103, the user terminals may transmit data signals over uplinks according to the scheduling information received from the base station (operation S104).
After operation S104, the base station may decide whether or not to perform a hybrid ARQ (automatic repeat request) according to whether or not the data signals received from the user terminals were properly decoded (operation S105).
With this type of scheduling scheme based on the related art, many uplink reference signals may be needed, since the uplink reference signals are transmitted before the scheduling, and consequently the spectral efficiency of the overall system may be decreased.
Also, since the uplink reference signals are transmitted before the scheduling, the channel information of user terminals for which scheduling was not performed is either not retained or not utilized, resulting in lowered energy efficiency of the user terminals.
Moreover, as the transmissions follow the sequence of uplink→downlink→uplink, there may occur two uplink/downlink transmission conversions, which may cause considerable delays in the context of TDD (time-division duplexing) or FDD (frequency-division duplexing).